Hemophilia A is a sex-linked bleeding disorder characterized by a deficiency in Factor VIII, an essential element in the blood coagulation cascade. The disease occurs in about 0.01% of the male population. Hemophilia A can be treated by administering Factor VIII-containing blood plasma obtained from healthy donors. This treatment has several disadvantages however. The supply of Factor VIII is limited and very expensive; the concentration of Factor VIII in blood is only about 100 ng/ml and the yields using current plasma fractionation methods are low. Since the source of Factor VIII is pooled donor blood, the recipient runs a high risk of acquiring various infectious diseases, including those caused by hepatitis non-A, non-B, hepatitis B or AIDS viruses which may be present in the donor blood. In addition, recipients may develop antibodies against the exogenous Factor VIII, which can greatly reduce its effectiveness.
Factor VIII comprises three regions, an N-terminal region, the so-called "AlA2-domain"; a central region, the so-called "B domain"; and a C-terminal region comprising the A3, C1 and C2 domains. The A1A2-domain and the C-terminal region are believed essential for clotting activity. Factor VIII circulates in the blood combined with a protein, the von Willebrand factor (vWf), which is believed to protect the sensitive Factor VIII against early degradation.
Factor VIII is obtained in unsatisfactorily low yields when produced by known recombinant DNA processes. Moreover the proteins appear not to be present as an intact chain, hence products are difficult to isolate and to purify and consequently the costs are high. It is therefore desirable to develop an efficient way to produce large quantities of compounds having Factor VIII activity, the compounds preferably having decreased immunogenic activity.
Relevant Literature
Molecular cloning of Factor VIII cDNA obtained from human mRNA and the subsequent production of proteins with Factor VIII activity in mammalian, yeast and bacterial cells has been reported. See WO 85/01961, EP 160457, EP 150735, EP 0253455. A method for producing proteins with Factor VIII activity using transformed microorganisms is disclosed in EP 0253455. European patent applications EP 150735 and EP 123945 and Brinkhous et al., Proc. Natl. Acad. Sci. USA (1985) 82:8752-8756 disclose Factor VIII activity in proteolytic cleavage products of Factor VIII. A complex of two proteolytic cleavage products of Factor VIII, a 92 kDa and an 80 kDa polypeptide, exhibits enhanced Factor VIII activity. (Fay et al., Biochem. Biophys. Acta (1986) 871:268-278; Eaton et al., Biochemistry (1986) 25:505-512).
Eaton et al., Biochemistry (1986) 25:8343-8347 disclose that a polypeptide in which 766 amino acids (797 through 1562) have been deleted from the central region retains Factor VIII activity. Moreover, mammalian cells transformed with a vector comprising DNA encoding this deletion polypeptide had a higher production level than cells transformed with a vector comprising DNA encoding the full length polypeptide.
PCT application WO 86/06101 discloses that recombinant Factor VIII proteins with deletions of up to 880 amino acids in the central region exhibit Factor VIII activity. The largest deletion stretches from T-760 through N-1639 (numbering according to FIG. 1). The host cells for preparation of the recombinant Factor VIII included mammalian cells, for example Chinese hamster ovary cells.